Slow light in paraffin-coated Rb vapor cells

We present preliminary results from an experimental study of slow light in anti-relaxation-coated Rb vapor cells, and describe the construction and testing of such cells. The slow ground state decoherence rate allowed by coated cell walls leads to a dual-structured electromagnetically induced transparency (EIT) spectrum with a very narrow (<100 Hz) transparency peak on top of a broad pedestal. Such dual-structure EIT permits optical probe pulses to propagate with greatly reduced group velocity on two time scales. We discuss ongoing efforts to optimize the pulse delay in such coated cell systems.Comment: 6 pages, 6 figures, submitted to Journal of Modern Optic

Three-dimensional coherent X-ray diffraction imaging via deep convolutional neural networks

As a critical component of coherent X-ray diffraction imaging (CDI), phase retrieval has been extensively applied in X-ray structural science to recover the 3D morphological information inside measured particles. Despite meeting all the oversampling requirements of Sayre and Shannon, current phase retrieval approaches still have trouble achieving a unique inversion of experimental data in the presence of noise. Here, we propose to overcome this limitation by incorporating a 3D Machine Learning (ML) model combining (optional) supervised learning with transfer learning. The trained ML model can rapidly provide an immediate result with high accuracy which could benefit real-time experiments, and the predicted result can be further refined with transfer learning. More significantly, the proposed ML model can be used without any prior training to learn the missing phases of an image based on minimization of an appropriate ‘loss function’ alone. We demonstrate significantly improved performance with experimental Bragg CDI data over traditional iterative phase retrieval algorithms

Identification and genomic location of a reniform nematode (Rotylenchulus reniformis) resistance locus (Renari) introgressed from Gossypium aridum into upland cotton (G. hirsutum)

In this association mapping study, a tri-species hybrid, [Gossypium arboreum × (G. hirsutum × G. aridum)2], was crossed with MD51ne (G. hirsutum) and progeny from the cross were used to identify and map SSR markers associated with reniform nematode (Rotylenchulus reniformis) resistance. Seventy-six progeny (the 50 most resistant and 26 most susceptible) plants were genotyped with 104 markers. Twenty-five markers were associated with a resistance locus that we designated Renari and two markers, BNL3279_132 and BNL2662_090, mapped within 1 cM of Renari. Because the SSR fragments associated with resistance were found in G. aridum and the bridging line G 371, G. aridum is the likely source of this resistance. The resistance is simply inherited, possibly controlled by a single dominant gene. The markers identified in this project are a valuable resource to breeders and geneticists in the quest to produce cotton cultivars with a high level of resistance to reniform nematode

Tm-values and unfolded fraction can predict aggregation rates for GCSF variant formulations, but not under predominantly native conditions.

Protein engineering and formulation optimisation strategies can be taken to minimise protein aggregation in the biopharmaceutical industry. Short-term stability measures such as the mid-point transition temperature (Tm) for global unfolding provide convenient surrogates for longer-term (eg 2-year) degradation kinetics, with which to optimise formulations on practical time-scales. While successful in some cases, their limitations have not been fully evaluated or understood. Tm values are known to correlate with chemical degradation kinetics for wild-type granulocyte colony stimulating factor (GCSF) at pH 4-5.5. However, we found previously that the Tm of an antibody Fab fragment, only correlated with its rate of monomer loss at temperatures close to the Tm. Here we evaluated Tm, the fraction of unfolded protein (fT) at temperature T, and two additional short-term stability measures, for their ability to predict the kinetics of monomer and bioactivity loss of wild-type GCSF and four variants, at 37 °C, and in a wide range of formulations. The GCSF variants introduced one to three mutations, giving a range of conformational stabilities spanning 7.8 kcal mol-1. We determined the extent to which the formulation rank order differs across the variants, when evaluated by each of the four short-term stability measures. All correlations decreased as the difference in average Tm between each pair of GCSF variants increased. The rank order of formulations determined by Tm was the best preserved, with R2-values >0.7. Tm-values also provided a good predictor (R2 = 0.73) of the aggregation rates, extending previous findings to include GCSF variant-formulation combinations. Further analysis revealed that GCSF aggregation rates at 37 °C, were dependent on the fraction unfolded at 37 °C (fT37), but transitioned smoothly to a constant baseline rate of aggregation at fT37 <10-3. A similar function was observed previously for A33 Fab formulated by pH, ionic strength and temperature, without excipients. For GCSF, all combinations of variants and formulations fit onto a single curve, suggesting that even single mutations destabilised by up to 4.8 kcal mol-1, are insufficient to change significantly the baseline rate of aggregation under native conditions. The baseline rate of aggregation for GCSF under native conditions, was 66-fold higher than that for A33 Fab, highlighting that they are a specific feature of each native protein structure, likely to be dependent on local surface properties and dynamics

Structure of a seeded palladium nanoparticle and its dynamics during the hydride phase transformation

Palladium absorbs large volumetric quantities of hydrogen at room temperature and ambient pressure, making the palladium hydride system a promising candidate for hydrogen storage. Here, we use Bragg coherent diffraction imaging to map the strain associated with defects in three dimensions before and during the hydride phase transformation of an individual octahedral palladium nanoparticle, synthesized using a seed-mediated approach. The displacement distribution imaging unveils the location of the seed nanoparticle in the final nanocrystal. By comparing our experimental results with a finite-element model, we verify that the seed nanoparticle causes a characteristic displacement distribution of the larger nanocrystal. During the hydrogen exposure, the hydride phase is predominantly formed on one tip of the octahedra, where there is a high number of lower coordinated Pd atoms. Our experimental and theoretical results provide an unambiguous method for future structure optimization of seed-mediated nanoparticle growth and in the design of palladium-based hydrogen storage systems

Imaging the Phase Transformation in Single Particles of the Lithium Titanate Anode for Lithium-Ion Batteries

Lithium uptake and release in lithium titanate (LTO) anode materials during a discharge and charge cycle is one of the fundamental processes of a lithium-ion battery (LIB), still not fully understood at the microscopic level. During the discharge cycle, LTO undergoes a phase transformation between Li4Ti5O12 and Li7Ti5O12 states within a cubic crystal lattice. To reveal the details of the microscopic mechanism, it is necessary to track the sequence of phase transformations at different discharge/charge states under operating conditions. Here, we use in situ Bragg coherent diffraction imaging (BCDI) and in situ X-ray diffraction (XRD) experiments to examine the lithium insertion-induced materials phase transformation within a single LTO particle and a bulk battery analogue, respectively. BCDI analysis from (111) Bragg peak shows the two-phase transformation manifesting as a distinct image phase modulation within a single LTO nanoparticle occurring in the middle of the discharge region then subsiding toward the end of the discharge cycle. We observe the biggest phase variation at the two-phase stage, indicating the formation of phase domains of 200 nm in size during the discharge process. We also observe a lattice contraction of >0.2% in a single LTO nanoparticle at the (400) Bragg peak measurement, larger than that in the corresponding bulk material. Our observation of this phase transformation at a single-particle level has implications for the understanding of the microscopic/mesoscale picture of the phase transformation in anode and cathode LIBs materials

Structural Explanation of the Dielectric Enhancement of Barium Titanate Nanoparticles Grown under Hydrothermal Conditions

When synthesized under certain conditions, barium titanate (BaTiO3, BTO) nanoparticles are found to have the non-thermodynamic cubic structure at room temperature. These particles also have a several-fold enhanced dielectric constant, sometimes exceeding 6000, and are widely used in thin-layer capacitors. A hydrothermal approach is used to synthesize BTO nanocrystals, which are characterized by a range of methods, including X-ray Rietveld refinement and the Williamson–Hall approach, revealing the presence of significant inhomogeneous strain associated with the cubic phase. However, X-ray pair distribution function measurements clearly show the local structure is lower symmetry than cubic. This apparent inconsistency is resolved by examining 3D Bragg coherent diffraction images of selected nanocrystals, which show the existence of ≈50 nm-sized domains, which are interpreted as tetragonal twins, and yet cause the average crystalline structure to appear cubic. The ability of these twin boundaries to migrate under the influence of electric fields explains the dielectric anomaly for the nanocrystalline phase

Anisotropy of antiferromagnetic domains in a spin-orbit Mott insulator

The temperature-dependent behavior of magnetic domains plays an essential role in the magnetic properties of materials, leading to widespread applications. However, experimental methods to access the three-dimensional (3D) magnetic domain structures are very limited, especially for antiferromagnets. Over the past decades, the spin-orbit Mott insulator iridate Formula Presented has attracted particular attention because of its interesting magnetic structure and analogy to superconducting cuprates. Here, we apply resonant x-ray magnetic Bragg coherent diffraction imaging to track the real-space 3D evolution of antiferromagnetic ordering inside a Formula Presented single crystal as a function of temperature, finding that the antiferromagnetic domain shows anisotropic changes. The anisotropy of the domain shape reveals the underlying anisotropy of the antiferromagnetic coupling strength within Formula Presented. These results demonstrate the high potential significance of 3D domain imaging in magnetism research

Evolution of ferroelastic domain walls during phase transitions in barium titanate nanoparticles

In this work, ferroelastic domain walls inside BaTiO3 (BTO) tetragonal nanocrystals are distinguished by Bragg peak position and studied with Bragg coherent x-ray diffraction imaging (BCDI). Convergence-related features of the BCDI method for strongly phased objects are reported. A ferroelastic domain wall inside a BTO crystal has been tracked and imaged across the tetragonal-cubic phase transition and proves to be reversible. The linear relationship of relative displacement between two twin domains with temperature is measured and shows a different slope for heating and cooling, while the tetragonality reproduces well over temperature changes in both directions. An edge dislocation is also observed and found to annihilate when heating the crystal close to the phase transition temperature

Author Correction: Three-dimensional strain imaging of irradiated chromium using multi-reflection Bragg coherent diffraction

The original version of this Article did not correctly credit and cite relevant previous work. The fifth to seventh sentences of the fifth paragraph of the ‘Three-dimensional imaging of the defects’ section previously read: “In our case, BCDI is sensitive to defects such as voids and dislocations through its strain field sensitivity rather than the spatial resolution46. This is illustrated by the relationship between the continuum representation of the crystal, (Formula presented.) , and the diffraction intensity, I(q) in the far field under a perfectly coherent illumination and in the kinematical scattering approximation given by (Formula presented.). Here, r and q are the real and reciprocal space coordinates respectively, (Formula presented.) is the Fourier transform, Q is the measured Bragg peak, and u(r) is the vector displacement field that is a continuum description of how the atoms are displaced from their equilibrium positions47.” The correct version reads: “In our case, BCDI is sensitive to defects such as voids and dislocations through its strain field sensitivity rather than the spatial resolution46. This is demonstrated by the relationship (Formula presented.). whereby (Formula presented.) is the intensity, (Formula presented.) is the mathematical description of the crystal as a continuum, (Formula presented.) denotes the Fourier transformation operator, Q is the Bragg reflection that was measured, and u(r) is the displacement field47.” The final six sentences of the Results section previously read: “Furthermore, underestimating the defect density prevents TEM from accurately determining the corresponding change in properties. For instance, Weiß et al. show a factor of 2 between measured and calculated change in hardness for neutron irradiated EUROFER9771. Meanwhile, Reza et al. report the same discrepancy between Transient Grating Spectroscopy (TGS)-measured and TEM-determined thermal diffusivity for self-ion irradiated tungsten72. It is important to note that when Reza et al. included small defects from molecular dynamics (MD) simulations, the combination of the TEM and MD data matches TGS measurements. This result confirms the theory that point defects play a significant role in the thermal diffusivity of a material and further reinforces the need to accurately characterize small defects in order to evaluate irradiation-induced changes in properties.” This has been replaced with: ““Hirst et al. opined that the underestimated defects density in TEM measurements comes with a corresponding mischaracterization of the materials properties70. This is demonstrated in a study by Weiß et al. who showed that the hardness values obtained from TEM data of neutron irradiated reduced activation ferritic/martensitic steel is significantly smaller than values from tensile testing. This clearly support the notion that underestimation of point defects from TEM analysis which goes into the dispersed barrier hardening model affects the calculated hardness value71. Hence, the difference in the magnitude of swelling between TEM and BCDI estimates is well justified. In a bid to accurately quantify nanoscale defects in irradiated materials, Meslin et al., used multiple characterization techniques which include TEM, Small Angle Neutron Scattering, Positron Annihilation Spectroscopy and Atom Probe Tomography which are sensitive to different types of nanoscale defects. The study clearly demonstrates the strength and complementarities of each technique72. This further support the need to develop multiple characterization techniques that can complements TEM for defects quantification and building predictive tools.” Consequently, Reference 72, which previously read “Reza, A., Yu, H., Mizohata, K. & Hofmann, F. Thermal diffusivity degradation and point defect density in self-ion implanted tungsten. Acta Mater. 193, 270–279 (2020)”, has been replaced by “Meslin, E. et al. Characterization of neutron-irradiated ferritic model alloys and a RPV steel from combined APT, SANS, TEM, and PAS analyses J. Nucl. Mater. 406, 73–83 (2010).” This has been corrected in both the PDF and HTML versions of the Article